The present invention relates generally to demodulation reference signals (DM-RSs) for LTE and LTE advanced communication systems and, more particularly, to the configuration of antenna ports for user-specific DM-RSs.
The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of UMTS (Universal Mobile Telecommunication Service) system and LTE (Long Term Evolution). LTE is a communication technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, which is thought as a next generation mobile communication system of the UMTS system. The 3GPP work on LTE is also referred to as E-UTRAN (Evolved Universal Terrestrial Radio Access Network). The first release of LTE, referred to as release-8 (Rel-8) can provide peak rates of 100 Mbps, a radio-network delay of, e.g., 5 ms or less, a significant increase in spectrum efficiency and a network architecture designed to simplify network operation, reduce cost, etc. In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes. The modulation technique or the transmission scheme used in LTE is known as OFDM (Orthogonal Frequency Division Multiplexing).
For the next generation mobile communications system, e.g., IMT-advanced (International Mobile Telecommunications) and/or LTE-advanced, which is an evolution of LTE, support for bandwidths of up to 100 MHz is being discussed. LTE-advanced can be viewed as a future release of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-advanced can be deployed in spectrum already occupied by LTE. In both LTE and LTE-advanced radio base stations known as evolved NodeBs (eNBs or eNodeBs), multiple-input, multiple output (MIMO) antenna configurations and spatial multiplexing can be used in order to provide high data rates to user terminals. Another example of a MIMO-based system is WiMAX (Worldwide Interoperability for Microwave Access) system.
To carry out coherent demodulation of different downlink physical channels, the user terminal needs estimates of the downlink channel. More specifically, in the case of OFDM transmissions, the user terminal needs an estimate of the complex channel of each subcarrier. One way to enable channel estimation in the case of OFDM transmissions is to insert known reference symbols into the OFDM time frequency grid. In LTE, these reference symbols are jointly referred to as downlink reference signals.
Two types of downlink reference signals are used in LTE systems: cell specific downlink reference signals and user specific downlink reference signals. Cell specific downlink reference signals are transmitted in every downlink subframe, and span the entire downlink cell bandwidth. The cell specific reference signals can be used for channel estimation and coherent demodulation except when spatial multiplexing is used. A user terminal specific reference signal is used for channel estimation and demodulation of the downlink shared channel when spatial multiplexing is used. The user specific reference signals are transmitted within the resource blocks assigned to the specific user terminal for transmitting data on the downlink shared channel. The user terminal specific reference signals are subject to the same precoding as data signals transmitted to the user terminal. The present invention is applicable to user terminal specific downlink reference signals.
FIG. 1 illustrates a portion of an exemplary OFDM time-frequency grid 50 for LTE. Generally speaking, the time-frequency grid 50 is divided into one millisecond subframes. One subframe is shown in FIG. 1. Each subframe includes a number of OFDM symbols. For a normal cyclic prefix (CP) link, suitable for use in situations where multipath dispersion is not expected to be extremely severe, a subframe comprises fourteen OFDM symbols. A subframe comprises twelve OFDM symbols if an extended cyclic prefix is used. In the frequency domain, the physical resources are divided into adjacent subcarriers with a spacing of 15 kHz. The number of subcarriers varies according to the allocated system bandwidth. The smallest element of the time-frequency grid 50 is a resource element. A resource element comprises one OFDM symbol on one subcarrier.
For purposes of scheduling transmission on the downlink shared channel (DL-SCH), the downlink time-frequency resources are allocated in units called resource blocks (RBs). Each resource block spans twelve subcarriers (which may be adjacent or distributed across the frequency spectrum) and one-half of one subframe. The term “resource block pair” refers to two consecutive resource blocks occupying an entire one millisecond subframe.
Certain resource elements within each subframe are reserved for the transmission of downlink reference signals. FIG. 1 illustrates one exemplary resource allocation pattern for the downlink reference signals to support downlink transmissions up to rank 4. Twenty-four resource elements within a subframe are reserved for transmission of the downlink reference signals. More specifically, the demodulation reference signals are carried in OFDM symbols 5, 6, 12, and 13 (i.e., the sixth, seventh, thirteenth, and fourteenth symbols) of the OFDM subframe. The resource elements for the demodulation reference signals are distributed in the frequency domain.
The resource elements for the demodulation reference signals are divided into two code division multiplexing (CDM) groups referred to herein as CDM Group 1 and CDM Group 2. In LTE systems supporting transmission ranks from 1-4, two CDM groups are used in combination with length-2 orthogonal cover codes (OCCs). The orthogonal cover codes are applied to clusters of two reference symbols. The term “cluster” as used herein refers to groupings of adjacent (in the time domain) reference symbols in the same subcarrier. In the embodiment shown in FIG. 1, the subcarriers containing demodulation reference symbols include two clusters each.
FIG. 2 illustrates an exemplary allocation of resource elements for a spatial multiplexing system supporting transmission ranks up to eight. It may be noted that the resource allocation pattern is the same as the allocation pattern shown in FIG. 1. To support higher transmission ranks, a length-4 OCC is used instead of a length-2 OCC. The length-4 OCC is applied across two clusters of resource elements.
Up to eight antenna ports may be defined to support up to 8 spatial layers. The 8 antenna ports can be mapped to two CDM groups, each using four OCCs. Thus, the antenna ports can be uniquely identified by two parameters, i.e., CDM group index and OCC index, referred to herein as an index pair. Currently, the mapping between antenna ports and index pairs has not been specified in the LTE standard. Some mappings may be rank dependent, which requires that different port mappings be used for each transmission rank. Using different port mappings for different transmission ranks imposes a burden on the user terminal, which must perform channel estimation differently when the transmission ranks changes.